Economic factors dictate the use of multi-party lines under circumstances where, as in some rural areas, there are a limited number of subscribers in a given locale. This multi-party approach provides for the use of one pair of wires by many customers and can be accomplished by the use of party lines or the use of prior art subscriber carrier systems wherein each subscriber is assigned a particular frequency spectrum for his transmitter and another spectrum for his receiver. The obvious advantage of the subscriber carrier approach is that each subscriber can use his telephone independently of others using the same pair of wires, thereby providing private service over the shared wires. In general, prior art subscriber carrier systems utilize a separate spectrum for transmitting and receiving for each subscriber. Thus, a five subscriber system requires ten spectral bands. Because of cable losses and crosstalk, the upper frequency is limited to about 150 KHz. Due to these constraints, for conventional systems now in common use, the number of subscribers is generally limited to about five to eight per pair of wires. Thus, in order to service twelve subscribers, two or three pairs of wires would be required.
The present invention concerns the provision of a system which enables transmission in both directions in the same spectrum and thus allows doubling of the number of customers as compared with conventional systems without the use of any additional lines. Thus, in this system, a five subscriber set-up requires five spectral bands only.
An approach developed independently of the present invention but bearing some broad similarity thereto is disclosed in U.S. Pat. No. 3,822,366 (O'Dea et al). This patent concerns a one channel carrier intercom system wherein transmission and reception take place in the same spectrum but not simultaneously. The system is intended for use by telephone men for conversation on an in-use physical pair of wires, rather as a full carrier system. The carrier is sent to the receiver by a simplex connection of the wire pair. The carrier is applied between ground and the wire pair so that the receiver recovers the carrier between the wire pair and ground, and recovers the signal across the pair. A diode ring acts as both the modulator and demodulator in this system. It will be evident from the description of the present invention set forth hereinbelow that the system of the present invention is quite different from that of the O'Dea et al patent.